The electric double layer capacitor has such a principle that positive and negative surfaces of polarized electrodes physically adsorb anion and cation of electrolyte for storing electric energy.
A bipolar type electric double layer capacitor (which will be called just as capacitor hereinafter) is a stacked device that comprises stacked flat activated carbon electrodes and collector electrodes, and ion passing separators each being sandwiched between adjacent two activated carbon electrodes. Around a unit of the activated carbon electrodes and collector electrodes, there are sealingly put packing members for preventing leakage of a liquid electrolyte therefrom. The packing members serve also as means for achieving isolation between the layers.
In assembling the capacitor, cells by an amount that ensures a needed withstand voltage (cell is the minimum unit of the electrolyte put between mutually opposed activated carbon electrodes) and the packing members are alternately put on one another (each cell having a withstand voltage of about 2.5V), to constitute a stacked body and paired endplates are respectively put on opposed portions of the body. With this, a hermetically sealed construction is provided.
For achieving a perfect sealing between cells, it is necessary to apply a sufficient fastening force to the paired endplates. For this purpose, several pairs of metal screws are passed through openings of the paired endplates, and spacers each having opposed ends with threaded openings are put between the paired endplates, and each pair of metal screws are screwed into the threaded openings of each spacer, so that a sufficient fastening force is applied to the paired endplates.
Furthermore, in order to prevent the entry of water for avoiding mixing of the liquid electrolyte in the capacitor with water, the capacitor is entirely covered with an aluminum laminate film, the interior of the aluminum laminate film thus shaped like a bag and the interior of the capacitor are decompressed to provide the interiors with a vacuum condition, and the aluminum laminate film is welded to the capacitor. With these steps, the capacitor is completely covered and sealed by the aluminum laminate film. This complete covering and sealing by the film is aimed for suppressing quality deterioration and gas generation.
Since the aluminum laminate film is a substance that consists of an aluminum foil laminated with a polypropylene film, polyethylene or the like, the same tends to be easily broken when an external shock is applied thereto, such as when the film is brought into collision with a desk or the like. In such case, moisture in air enters the capacitor and thus the quality of the capacitor is lowered.
As means for protecting the capacitor from such external shock, Japanese Laid-open Patent Application (Tokkai) 2004-296956 discloses a protecting case that entirely covers both the external surface (especially corner portions) of the capacitor and the bag-shaped aluminum laminate film.